Anti-cytokine therapy for acute and chronic inflammatory diseases has entered clinical medicine. The main targets for anti-cytokine-based therapies are tumor necrosis factor (TNF) and interleukin-1 (IL-1), pleiotropic, proinflammatory cytokines. IL-1B is synthesized as a precursor requiring a protease called IL-1B converting enzyme (ICE) for cleavage and secretion of active IL-1B. A novel cytokine called interferon-gamma-inducing factor (now named IL-18), also requiring ICE for cleavage and secretion to an active cytokine, is the primary objective for the present study. Specific inhibitors of ICE reduce the release and hence the biological activity of both IL-1B and IL-18. Mature IL-18 is structurally similar to mature IL-1B. Initially reported as a co-stimulant of interferon-gamma (IFNg) production in mice during endotoxemia, preliminary studies demonstrate that IL-18 has broad biological effects similar to those of IL-1B such as inducing the synthesis of other pro-inflammatory cytokines and activation of nuclear factor kappa-B. Little is known about the production and biological properties of IL-18. The current proposal focuses on the nature of the IL-18 receptor signaling complex. We have purified from human urine a soluble IL-18 binding protein which specifically neutralizes the biological activity of IL-18 and likely presents the extracellular domain of the IL-18 ligand binding receptor. Amino acid sequencing of this IL-18 binding resulted in N-terminal 40 amino acids which are a perfect match to a partial cDNA of 600 bp recently deposited in the TIGR data base. Using these 600 bp as a probe, we have observed a 1.5 and 4.2 transcript upon Northern hybridization. We propose to isolate cDNA clones for these two transcripts and demonstrate that they are an integral and essential component of the biological response signaled by IL-18. Antibodies to the IL-18 ligand will be used to reveal the role of IL-18 in animal models of disease, particularly inflammatory and autoimmune diseases. Antibodies to the ligand binding soluble receptor will be used to reveal the surface IL-18 receptor complexes. Targeted disruption of the IL-18 ligand-binding receptor will be carried out in mice with the goal of assessing the biological role IL-18 in health and disease. In these studies, we propose to unravel the nature of the IL-18 cell surface signaling complex and to examine whether this cytokine contributes to inflammatory disease. These studies will broaden the present concepts of proinflammatory cytokines in pathological processes.